Reprogramming of intestinal glucose metabolism and glycemic control in rats after gastric bypass

Abstract

The resolution of type 2 diabetes after Roux-en-Y gastric bypass (RYGB) attests to the important role of the gastrointestinal tract in glucose homeostasis. Previous studies in RYGB-treated rats have shown that the Roux limb displays hyperplasia and hypertrophy. Here, we report that the Roux limb of RYGB-treated rats exhibits reprogramming of intestinal glucose metabolism to meet its increased bioenergetic demands; glucose transporter-1 is up-regulated, basolateral glucose uptake is enhanced, aerobic glycolysis is augmented, and glucose is directed toward metabolic pathways that support tissue growth. We show that reprogramming of intestinal glucose metabolism is triggered by the exposure of the Roux limb to undigested nutrients. We demonstrate by positron emission tomography-computed tomography scanning and biodistribution analysis using 2-deoxy-2-[18F]fluoro-D-glucose that reprogramming of intestinal glucose metabolism renders the intestine a major tissue for glucose disposal, contributing to the improvement in glycemic control after RYGB.

Fig. 1. RYGB improves glycemic control

(A) Schematic drawing of RYGB (left). Intraoperative picture of RYGB in rats (right). RYGB in rats closely resembles the procedure performed in humans. The stomach (S) is divided, and a small gastric pouch is created (GP). The jejunum (J) is transected, and the distal part is brought up and connected through a gastro-jejunostomy (GJ) to the GP [this jejunal loop is called the Roux limb (RL)]. The continuation of the gastrointestinal (GI) tract is reestablished by reconnecting at the jejuno-jejunostomy (JJ), the proximal part of the jejunum further down to the RL [this is called the biliopancreatic limb (BP) because it drains the gastric, hepatic, and pancreatic secretions]. The part of the small intestine distal to the JJ is called the common limb (CL). RYGB reconfigures the GI tract and alters the flow of nutrients; nutrients flow from the esophagus (E) to the GP and then to the RL directly, bypassing the distal stomach (DS), the duodenum (D), and part of the proximal jejunum. Thus, the RL is exposed to undigested nutrients; the BP is exposed to the gastric, hepatic, and pancreatic secretions but no nutrients; and the CL is exposed to a mixture of nutrients with the gastric, hepatic, and pancreatic secretions. (B) Blood glucose levels were lower in RYGB-treated rats in comparison with sham-operated rats, 7 days after diabetes was induced by the administration of STZ. (C) RYGB-treated GK rats exhibited better glucose excursion curves after oral glucose administration. The inset shows the area under the curve (AUC) of the oral glucose tolerance test. (D) There was no difference in the insulin tolerance test between RYGB-treated and sham-operated GK rats. The inset shows the AUC of the insulin tolerance test. [(B) to (D)] N = 5 to 7 rats, 2 months postoperatively; mean T SEM; *P < 0.05; unpaired t test.

Fig. 2. RYGB increases the gene and protein expression levels of key factors and enzymes involved in glucose and cholesterol metabolism in the Roux limb

(A to C) RNA and protein levels of glycolytic enzymes and G6PD were increased in the Roux limb of RYGB-treated rats. B2M, b2 microglobulin. (D) The enzymatic activity of G6PD was increased in the Roux limb of RYGB-treated rats. (E and F) RNA and protein levels of factors and enzymes involved in cholesterol biosynthesis and uptake were increased in the Roux limb of RYGB-treated rats. HMGCS1, 3-hydroxy-3-methylglutaryl-CoA synthase 1; HMGCR, 3-hydroxy-3-methylglutaryl-CoA reductase; MVD, mevalonate (diphospho) decarboxylase; CYP51A1, cytochrome P450, family 51, subfamily A, polypeptide 1 (lanosterol 14-a demethylase); SREBP2, sterol regulatory element binding protein 2; LDLR, low-density lipoprotein receptor. There was no difference in the protein levels of Niemann-Pick C1-like 1 (NPC1L1). (G) Representative images of Roux limb sections of RYGB-treated rats stained with antibodies against HMGCR and LDLR (scale bar, 100 mm). Left to right: villi, crypts, and muscular layer. HMGCR expression was increased in the villi, the crypt cells, and the muscular layer while LDLR expression was increased in the villi and in the ganglia of the muscular layer. [(A) to (G)] N = 7 to 9 rats; [(A) to (F)] 2 months postoperatively; the results were reproduced at 1 and 6 months postoperatively; (G) 2 months postoperatively; mean T SEM; *P < 0.05; [(A) and (E)] one sample t test; (D) unpaired t test.

Fig. 3. RYGB enhances intestinal GLUT-1 mediated basolateral glucose uptake and utilization, which contributes to the improved whole-body glucose disposal

(A) Representative images of whole-body [18F]FDG PET/CT scanning in RYGB-treated and sham-operated rats. [18F]FDG uptake is colorcoded, and areas of increased signal exhibit red-orange color. There was intense [18F]FDG uptake by the Roux limb (marked with an arrow) of RYGB-treated rats in comparison with the corresponding jejunum of sham-operated rats. The cross indicates the exact same point of the intestine in all images. (B and C) GLUT-1 RNA and protein levels were increased in the Roux limb of RYGB-treated rats. (D) Representative images of whole-body [18F]FDG PET/CT scanning in RYGBtreated rats, with or without the administration of phloretin, a GLUT-1 inhibitor. Phloretin substantially blunted [18F]FDG uptake by the Roux limb (marked with an arrow). The cross indicates the exact same point of the intestine in all images. (E and F) [18F]FDG biodistribution analysis in RYGB-treated and shamoperated rats demonstrated higher [18F]FDG uptake by the intestine of RYGB-treated rats. WAT, white adipose tissue; BAT, brown adipose tissue. The RL of RYGBtreated rats displayed the highest [18F]FDG uptake. The uptake by the CL of RYGB-treated rats was also increased. There was no difference in the [18F]FDG uptake between the BP of RYGB-treated rats and the corresponding intestinal segment of sham-operated rats. The [18F]FDG uptake was constant along the intestine of sham-operated rats. (G) Consistent with improved whole-body glucose disposal, RYGB-treated rats exhibited lower [18F]FDG signal in the blood. [(A) to (G)] N = 7 to 9 rats; [(A) to (C)] and [(E) to (G)] 2 months postoperatively; the results were reproduced at 1 and 6 months postoperatively; (D) 6 months postoperatively; mean T SEM; *P < 0.05; (B) one sample t test; [(E) to (G)] unpaired t test.

Fig. 4. Reprogramming of intestinal glucosemetabolism is triggered by the exposure of the Roux limb to undigested nutrients

(A) Schematic drawing (left) and intraoperative picture (right) of the ES-JLI rat model. A loop of jejunum is transected and transposed between the esophagus and the stomach without performing any other anatomic alterations of the RYGB procedure; that is, the stomach is not divided, the duodenum is not excluded, and the continuity of the gastrointestinal tract remains intact. Nutrients flow from the esophagus (E) through the esophago-jejunostomy (EJ) to the transposed jejunal loop (TJ) and then through the jejuno-gastrostomy (JG) to the stomach (S), the duodenum (D), the jejunum (J), and the rest of the gastrointestinal tract. (B) GLUT-1 and HK2 protein levels were increased in the transposed jejunal loop of ES-JLI–treated rats. (C) Representative images of whole-body [18F]FDG PET/CT scanning in ES-JLI–treated rats. [18F]FDG uptake is color-coded, and areas of increased signal exhibit red-orange color. There was intense [18F]FDG uptake by the transposed jejunal loop (marked with an arrow) of ES-JLI–treated rats. The cross indicates the exact same point of the intestine in all images. [(B) and (C)] N = 5 to 7 rats, 1 month postoperatively; the results were reproduced at 6 months postoperatively.